Demands for increased areal recording density in magnetic disk drives in turn require read heads with a higher signal-to-noise ratio (SNR). Demands for a higher SNR increase the need to shield the sensor element from external signals such as RF interference or noise.
A typical prior art head and disk system is illustrated in FIG. 1. In operation the head 10 is supported by a suspension 13 as it flies above the disk 16. The magnetic transducer, usually called a “head,” is composed of elements that perform the task of writing magnetic transitions (the write head 23) and reading the magnetic transitions (the read head 12). The electrical signals to and from the read and write heads 12, 23 travel along conductive paths 14 which are attached to or embedded in the suspension arm. Typically there are two electrical contact pads each for the read and write heads. Wires or leads are connected to these pads and routed in the suspension 13 to the arm electronics (not shown). The disk 16 is attached to a spindle 18 which is driven by a spindle motor 24 to rotate the disk. The disk 16 comprises a substrate 26 on which a plurality of thin films 21 are deposited. The thin films include ferromagnetic material in which the write head records the magnetic transitions in which information is encoded.
The layers comprising the read head 12 of the prior art head 10 are further illustrated in FIG. 2. Only selected layers have been illustrated in the figure for clarity. The undercoat 11 is deposited on substrate 15. The sensor element 32, which can be a Giant Magnetoresistive (GMR) element or the like, is surrounded on two sides in a sandwich fashion by two magnetic shields 17, 19 which are typically called S1 and S2 respectively. A primary function of S1 and S2 is to shield the sensor element (GMR, etc.) from adjacent magnetic signals on the disk drive surface and therefore, to allow the sensor to respond only to the signal from a very small area of magnetic material on the disk and thereby reduce the bit size. The shields also protect the sensor element from the magnetic field generated by the write head.
Variations on the basic S1 and S2 shielding include magnetically connecting the shields through a low reluctance path as is described in U.S. Pat. No. 5,923,502 to Christensen, et al.
U.S. Pat. No. 5,754,369 to Balakrishnan provides electrostatic shielding during reading by grounding the write traces in the suspension which are routed proximate to the read traces.
A magnetic tunnel junction (MJT) magnetoresistive head is described in U.S. Pat. No. 5,898,548 to Dill, et al., which has electrically conducting spacer layers at the top and bottom of the MJT. These electrically conducting spacer layers lie between the shields and are used to connect the MJT to the shields.
A model of the equivalent circuit of the prior art read head interface with the disk shows three largest elements of capacitive coupling: S1 to disk; S2 to disk and substrate to disk. The value of the total capacitance is inversely proportional to the distance between the disk and the head elements and directly proportional to the areas of the conductive elements. The thin films on the disk are conductive and, therefore, may act as an antenna that injects electromagnetic interference into the head. Depending on the specific values of the capacitance and other components in the circuit, strong external RF signals from sources outside of the drive and RF signals generated by the drive electronics can be picked up by the read head. The RF noise coupled from the disk by the mechanism described above, is further capacitively coupled into the sensor (e.g. GMR) element and the lead structure. The coupling occurs through multiple paths, with the largest contribution coming from capacitive coupling between the substrate 15 and the sensor leads (not shown), as well as, through the path from the substrate to S1 and the sensor leads. This noise is indistinguishable from the sensor signal, and therefore, adversely affects the SNR.